1. Field of the Art
This invention relates to a novel thermoplastic elastomer comprising a crosslinked acrylic rubber component and a copolymer component of a methacrylate ester with a maleimide, which excels in heat resistance and oil resistance, is soft or flexible, has an increased tensile strength and shows an improved moldability, and to a process for producing such a thermoplastic elastomer.
2. Background Art
Thermoplastic elastomers show rubber elasticity in the range of temperatures at which they are used but, unlike the so-called vulcanized rubbers, provide a high-molecular or polymeric material capable of being meltmolded at elevated temperatures. In other words, thermoplastic elastomers have the properties of both vulcanized rubbers and thermoplastic resins. Due to this character, they have recently been in great demand and so mass-produced.
Various such thermoplastic elastomers, now commercially available, are generally broken down into olefin, styrene, vinyl chloride, urethane, ester and amide types, based on their chemical structures or compositions of components.
Because of lower total cost than vulcanized rubber, there is a demand to use such thermoplastic elastomers instead of vulcanized rubber products where they are likely to be exposed to high temperature and come into contact with oils and fats.
The thermoplastic elastomers so far known in the art have some heat resistance, however, they fail to meet the above demand due to their problems that some are poor in oil resistance (e.g. olefin type); some excel in oil resistance but are poor in heat resistance (e.g. vinyl chloride and urethane types); or some excel in resistance to heat and oils in a hard region but are poor in resistance to oils and heat in a soft region like vulcanized rubbers (e.g. ester and amide types).
On the other hand, copolymers of alkyl esters of acrylic acid with a small amount of other monomer such as chloroethyl vinyl ether or ethylidene norbornene--which provides crosslinking points during vulcanization--are well known as acrylic rubbers, which are kneaded with a crosslinking agent by rolls or the like for practical use.
Further, it is known from Japanese Patent Publication No. 43(1968)-9753, that copolymers obtained by the copolymerization of maleimides with methyl methacrylate are superior in heat resistance to homopolymers of methyl methacrylate.
Also well-known are compositions of copolymers of methyl methacrylate with maleimides, to which acrylic rubber is added to improve the impact resistance of said copolymers (see, for instance, Japanese Patent Publication No. 43(1968)-9753 and Japanese Patent Kokai Koho No. 62(1987)-132911).
These conventional compositions, however, all fail to have both heat resistance and oil resistance in a soft region, partly because the content of acrylic rubber is limited.
Japanese Patent Kokai No. 62(1987)-132911 teaches that difficulty is encountered in the copolymerization of methyl methacrylate with maleimides at an acrylic rubber content of higher than 50% and that the resulting copolymer compositions, if obtained, are poor in heat resistance.
Japanese Patent Kokai No. 62(1987)-209113 teaches that the polymerization of a mixture of methyl methacrylate, an N-substituted maleimide, an alkyl acrylate and a difunctional monomer yields an interpolymer having both a glass transition temperature based on the alkyl polyacrylate and a glass transition temperature based on the copolymer of methyl methacrylate with the N-substituted maleimide. However, since it is taught that the polymeric moiety corresponding to the polyacrylic ester of this interpolymer should have a molecular weight in the range of 150,000 to 500,000, as measured by GPC, it is obvious that this polyacrylic ester component is soluble in a solvent and is not crosslinked.
Moreover, as illustrated in FIG. 2, a confirmatory experimentation carried out by the inventors (see Comparative Example 1 referred to later) also teaches that, where the acrylic ester content exceeds 50%, the single-stage polymerization effected according to what is set forth in said publication yields a polymer product found to have a single glass transition temperature lying midway between the glass transition temperature (Tg) of the copolymer of methyl methacrylate with the N-substituted maleimide and the Tg of the polyacrylic ester, as determined by the measurement of its viscoelasticity. Thus, that polymer product does not behaves as a thermoplastic elastomer at all.
Thus, in the above-mentioned prior art, there is no teaching or suggestion of thermoplastic elastomers comprising a crosslinked acrylic rubber component and a copolymeric component of a methacrylate ester with a maleimide, which have both heat resistance and oil resistance.
A novel thermoplastic elastomer which has both heat resistance and oil resistance not only in a relatively hard region but also in a soft vulcanized rubber region and has an improved moldability, as well as a process for its production, are still in great demand as an alternative to vulcanized rubber products for uses where it is likely to be exposed to elevated temperatures and come into contact with oils and fats.